The present invention relates to a blanket tensioning system for tensioning a blanket such as of woolen cloth, rubber sheet or the like around a blanket cylinder of an offset press.
The blanket stretched on the peripheral surface of the blanket cylinder is gradually elongated and becomes loose during the use of the press due to contact between an impression cylinder and a plate cylinder. It is, therefore, necessary to tension the blanket not only at the initial stage of operation but also during the long use of the press.
The conventional blanket tensioning systems can be broadly sorted into two types: namely one end adjustable type in which the tension is applied only to one end of the blanket while the other end is fixed, and two-end adjustable types which are also sorted into two types; namely a single spindle type in which both ends of the blanket are caught in a groove of a common spindle so that both ends are equally tensioned as the spindle is rotated, and two spindle type in which each end of the blanket is caught by each spindle independently of each other.
However, in the case of these spindle types, while both ends of the spindle are rotatably supported by brackets, the middle portion of the spindle bends under tensioning of the blanket, so that the middle portion of the blanket is more loose than the both end portions of the blanket.
On the other hand, since the groove in the peripheral surface of the blanket cylinder brings about an irregular printing whenever the groove rotatively contacts with the peripheral surface of the other cylinders, width of the groove should be minimized as much as possible in order to reduce the impacts occurring with the contacts.
In general, the loosening or slack of the blanket tends to appear in the trailing end portion rather than in the leading end portion, because the blanket cylinder in the press is usually rotated only in one direction. The portion of the blanket other than the trailing end portion is suitably tensioned due to the contact between the impression cylinder and the plate cylinder, and is held in good close contact with the peripheral surface of the blanket cylinder. It is, therefore, desirable that the tensioning is effected only in the portion near the trailing end of the blanket where the slack appears, without applying any tension to the portion which is held in good tension and close contact with the peripheral surface of the blanket cylinder. From this point of view, the one-end adjusting type system is preferred to the two-end adjusting type system mentioned before.
In recent years, however, there is a new requirement in this field of technic, in addition to the above-described demand. Namely, the use of such a press is becoming popular as having a pair of offset press units one of which is adapted to be rotated not only in the forward direction but also in the backward direction as desired, to permit a diversification of the printing specification such as capability of both of mono-color printing in both sides and two-color printing on a single side. In such a type of press, the leading end portion of the blanket which has been tensioned is changed into the trailing end portion in which the slack tends to appear, while the trailing end portion of the blanket which has been slackened is changed into the leading end portion of the blanket which is suitably tension, as the direction of rotation is changed. This gives a rise to a demand for a blanket tensioning system capable of dealing with such a change of state of tensioning of the blanket. Neither the one-end type tensioning system nor the two-end type tensioning system can cope with this demand.